Commentary. Monofunctional alkylating agents and apurinic sites in DNA

DNA-Protein Crosslink and DNA Strand Break Formation in HL-60 Cells Treated with Trans,trans-Muconaldehyde, Hydroquinone and Their Mixtures

The toxicity of benzene, a human leukemogen and ubiquitous environmental pollutant, is mediated in part by ring-hydroxylated metabolites including hydroquinone (HQ) and ring-opened metabolites including trans,trans-muconaldehyde (muconaldehyde, MUC), and their interactions. DNA-protein crosslinks (DNAPC) and DNA strand breaks (DNASB) are toxic lesions associated with the mechanism(s) of toxicity of carcinogenic compounds. In the present studies, we examined the hypothesis that individual and interactive effects of MUC and HQ are involved in the formation of DNAPC and DNASB. We extended our previous studies on DNAPC induction by MUC in HL-60 cells to HQ and mixtures of MUC and HQ, and determined DNASB levels, including 3'OH DNASB. Treatment of HL-60 cells with 25 to 100 μM HQ followed by incubation for 4 hours resulted in 1.3- to 2.8-fold increases in DNAPC levels compared with control, as determined by a K+/sodium dodecyl sulfate (SDS) precipitation assay. At 25 and 100 μM, MUC was 1.8 and 4.9 fold more effective at inducing DNAPC than HQ. Treatment with equimolar mixtures of 25 or 50 μM MUC and HQ resulted in higher DNAPC formation relative to the DNAPC levels expected if the effects were only additive. 3'OH DNASB levels as determined by the TUNEL assay showed a significant concentration-dependent increase 1 hour after treatment with 5 to 25 μM MUC, whereas HQ treatment had no effect. Cotreatment with 25 and 50 μM MUC/HQ mixtures resulted in significant decreases in TUNEL labeling relative to treatment with MUC alone. HL-60 cells treated with 1 to 50 μM MUC or HQ exhibited concentration- and time-dependent increases in DNASB as determined by the FADU assay, which measures a variety of single- and double-strand breaks and alkali labile sites. Exposure to 10 μM MUC gave Qdnasb values (1 Qdnasb ≍100 DNASB/cell) of 7.5 ± 1.2 and 15.4 ± 1.4 at the 1- and 2-hour time points respectively, compared with 0.1 ± 3.8 and 0.0 ± 1.5 for the corresponding time controls. The Qdnasb values after treatment with 10 μM HQ were 4.4 ± 0.7 and 17.7 ± 2.1 at the 1- and 2-hour time points, respectively, compared with 0.0 ± 0.5 and 0.0 ± 1.3 for the corresponding time controls. Induction of DNASB was additive 1 hour after treatment with equimolar MUC/HQ mixtures of 5 to 50 μM. These in vitro findings are significant in that DNAPC and DNASB lesions induced by MUC and HQ as well as their interactions could contribute to benzene-induced hematotoxicity and leukemogenesis.

Comparison of chemically induced DNA breakage in cellular and subcellular systems using the comet assay

The alkaline comet assay, employing a single cell gel electrophoresis, is a rapid, simple and sensitive technique for visualizing and measuring DNA damage leading to strand breakage in individual mammalian cells. In this report, we describe a modified version of this assay which we used to assess DNA damage as a result of treating lysed cells with genotoxic and antimetabolic agents. By means of this modified assay, DNA is no longer held under the regulation of any metabolic pathway or membrane barrier. Using 3 direct-acting agents, hydrogen peroxide, N-methyl-N-nitrosourea, and bleomycin, we were able to induce increased DNA migration by both the standard and modified comet assays. In contrast, with 4-nitroquinoline 1-oxide, 5-fluorouracil, and methotrexate, which require cellular enzymatic activity to induce DNA damage, we succeeded in inducing increased DNA migration using the standard comet assay conditions only. In some cases, the modified comet assay might be helpful in analyzing chemical and biological characteristics of genotoxic agents when performed in combination with the standard comet assay.

Mafosfamide induces DNA fragmentation and apoptosis in human T-lymphocytes. A possible mechanism of its immunosuppressive action

Cyclophosphamide, an alkylating agent belonging to the family of nitrogen mustards, is commonly used to treat progressive autoimmune diseases in humans. At the molecular level, its cytotoxicity results from DNA double strand crosslinks and, at higher concentrations, from DNA strand breaks. At the cellular level, cyclophosphamide may selectively affect mature lymphocytes with relative sparing of the respective precursor cells. In this study, we show that 4-hydroxycyclophosphamide (4-OH-CP), the active metabolite of cyclophosphamide, induces apoptosis in mature human lymphocytes at concentrations that are achieved in vivo. Since cyclophosphamide requires enzymatic conversion in the liver to yield its active metabolite, 4-OH-CP was generated in vitro by non-enzymatic hydrolysis of mafosfamide. Apoptotic cell death of lymphocytes was characterized by typical morphological changes, nucleosomal DNA fragmentation, and quantified by 3'-OH end labeling of fragmented DNA. The percentage of apoptotic cells both depended on drug concentration and time of exposure. Cycloheximide or ZnSO4 did not suppress 4-OH-CP induced apoptosis. Etoposide, a topoisomerase II inhibitor known to induce apoptosis in human tumor cell lines like 4-OH-CP, did induce detectable DNA fragmentation in only a minor proportion of T-lymphocytes but suppressed T-cell proliferation.

Effects of methyl methanesulfonate on mouse sperm chromatin structure and testicular cell kinetics

Effects of methyl methanesulfonate (MMS) on mouse testicular cell kinetics and sperm chromatin structure were determined flow cytometrically. Mice were exposed to a single ip injection of saline containing 0 or 150 mg/kg MMS. Relative ratios of 1N, 2N and 4N testicular cells were not affected until 22 days postexposure. Ratios of 1N cell types were altered from 13 to 22 days and were near normal by 25 days. This study revealed an MMS induced alteration of chromatin structure in testicular, elongated spermatids by the sperm chromatin structure assay (SCSA), a flow cytometric measure of the susceptibility of acridine orange stained sperm DNA to denaturation in situ. The SCSA also detected alterations in cauda sperm chromatin structure at 3 days, which was 8 days prior to alterations in sperm head morphology, indicating the increased sensitivity of the SCSA. SCSA data were practically similar whether measuring either fresh or frozen/thawed sperm, or whether measured by two different types of flow cytometers: a) laser driven, orthogonal optical axis; or b) low cost mercury arc lamp system with epiillumination. The data support the model of Sega and Owens [Mutat Res 111:227-244:1983] that MMS alkylates cysteine-SH groups in sperm protamines, thereby destabilizing sperm chromatin structure and leading to broken chromosomes and mutations.

Effects of benzene on DNA strand breaks in vivo versus benzene metabolite-induced DNA strand breaks in vitro in mouse bone marrow cells

Previously, we identified p-benzoquinone (BQ) and 1,2,4-benzenetriol (BT) as toxic metabolites of benzene on the basis of their inhibitory effect on DNA synthesis. In the present study, the capability of benzene and the two metabolites to induce DNA strand breaks was investigated in either the in vivo or the in vitro system by comparing the DNA elution rate on a fine membrane filter at alkaline pH. In the in vitro system were bone marrow cells were reacted with test chemicals for 60 min, both BQ and BT induced a dose-related increase in alkali-labile DNA single-strand breaks (SSBs) of bone marrow cells. However, when glutathione (350 micrograms/ml) was added to the same reaction system, the DNA damaging effect of BQ (24 microM) and BT (24 microM) was blocked by 100 and 53%, respectively. Catalase (130 units/ml) completely blocked the DNA damaging effect of BT, while no protection was afforded with BQ. Consistent with these observations, no induction of alkali-labile DNA SSBs was observed in the in vivo system by an anesthetic dose of benzene (1760 mg/kg, ip or po) at 1, 24, and 36 hr postadministration in both male and female ICR mice. These results suggest that benzene exposure would not induce direct DNA strand breaks in vivo under realistic work-related or accidental exposure conditions and also indicate that caution should be exercised in the interpretation of in vitro data for whole-body toxicity evaluation.

The relationship between DNA damage and mutation frequency in mammalian cell lines treated with N-nitroso-N-2-fluorenylacetamide

The induction of DNA single-strand breaks in C3H10T1/2 mouse fibroblasts and Chinese hamster ovary (CHO) cells by N-nitroso-N-2-fluorenylacetamide (N-NO-2-FAA) was demonstrated by the alkaline elution technique. Without metabolic activating system (i.e., rat liver S9 fraction), N-NO-2-FAA exhibits more direct and strong damaging effects on DNA than its parent compound, 2-FAA, at equal concentration in both cell lines. To compare the DNA-damaging potency of N-NO-2-FAA with other well-known carcinogens, such as benzo[a]pyrene, 2-nitrofluorene, and N-methyl-N'-nitrosoguanidine (MNNG), the order of potency is as follows: MNNG (5 microM) greater than N-NO-2-FAA (150 microM) greater than benzo[a]pyrene (20 microM) at equitoxic concentrations, LD37, in the same cell system. Another parallel experiment indicated that N-NO-2-FAA could disrupt the superhelicity of circular plasmid DNA (pBR 322) at a dose range of 0.1-50 mM; however, a complete conversion to form III linear DNA was found at the highest concentration (50 mM). After treatment with various concentrations of N-NO-2-FAA, ouabain resistance (ouar) was induced in C3H10T1/2 cells, while both ouar and 6-thioguanine resistance (6-TGr) were induced in CHO cells. The mutation frequency in the Na+/K+-ATPase locus in CHO cells (1.5 X 10(-6) mutants/microM) is higher than that in C3H10T1/2 cells (1.0 X 10(-6) mutants/microM). The maximal mutation frequency at the Na+/K+-ATPase gene locus was attained with 30 min of exposure in C3H10T1/2 cells, whereas the mutation frequency in CHO cells continued to increase up to 80 min of treatment. Similarly, the maximal mutation frequency at the HPRT locus also continued to increase up to 80 min of treatment. Finally, a linear plot of alkali-labile lesions versus 6-TGr mutations was obtained; but the same relationship was not observed in the case of ouar mutation.

Characterization of Escherichia coli mutant strains deficient in AP DNA-repair synthesis

Deficiency of apurinic/apyrimidinic (AP) DNA-repair enzymes in crude extracts of E. coli mutants was determined by following general and specific AP DNA-repair synthesis via nick translation in the presence of either all four dNTPs, or only one dNTP. We have shown that mutations either in DNA polymerase I or in AP endonucleases or in both, inhibit to different degrees the ability to repair AP DNA. The polA mutation totally abolishes the ability to perform both general and specific AP DNA repair, while the polAex mutation affects only general AP DNA repair. The xthA tight mutants, including the deletion mutant BW9101, can cope with small amounts of AP sites but hardly with high amounts of these lesions. In addition we have found that crude extracts of the xthA mutants degrade AP DNA by two modes: a nonspecific, and an AP-specific mode. These phenomena are common to all xth mutants and enabled us to discover this mutation. In contrast to the xth mutants so far isolated, BW2001 exhibits marked sensitivity to MMS and to X-ray irradiation. We found that this strain has a proficient DNA polymerase I but is absolutely deficient in AP endonucleases. We attribute its sensitivities to a secondary mutation at the structural gene of endonuclease IV.

DNA repair in Saccharomyces cerevisiae: purification and characterization of apurinic endonucleases

Five chromatographically distinct apurinic endonucleases (D1, D2, D3, D4, and E) were purified from Saccharomyces cerevisiae 234, 122, 1,000, 4,550, and 5,490-fold, respectively. All appeared to be class II apurinic endonucleases and were not contaminated with exonuclease or nonspecific endonuclease activities under the reaction conditions used. All had similar pH optima, but endonucleases D4 and E showed higher salt requirements and endonuclease D4 had a lower Mg2+ requirement for optimal activity than the other endonucleases. Endonuclease D4 also nicked OsO4-treated DNA. The molecular weights of the apurinic endonucleases as determined by glycerol gradient sedimentation analysis were 37,000, 49,000, and 10,000, for endonucleases E, D4, and D2, respectively. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of samples of radioiodinated endonuclease E showed the presence of two proteins.

Apurinic endonuclease activity remains constant during early Drosophila development

An endonuclease activity that acts on alkali-labile lesions in x-irradiated PM2 DNA and recognizes apurinic lesions in heat/acid treated DNA has been partially purified from Drosophila melanogaster embryos and its specific activity monitored throughout early development. The enzyme activity also showed a low level of activity on UV-irradiated DNA. The saturation kinetics observed with both x-irradiated and apurinic PM2 DNA substrates were similar. The endonuclease activity exhibited a broad pH optimum between pH 6 and 8.5 and was almost completely inhibited by 100 mM NaCl, 0.1 mM EDTA, 2 mM CaCl2 and 10 mM NEM. The reaction was not completely dependent on the presence of Mg++cation, but optimum activity was obtained at a concentration of 0.1 mM; concentrations greater than 1 mM Mg++ were inhibitory. The specific activity of the apurinic endonuclease, partially purified from several stages of embryonic and early larval development, remained the same. Unfertilized eggs exhibited a reduced level of this presumptive repair activity.

Methylation of DNA and protamine by methyl methanesulfonate in the germ cells of male mice

The molecular dosimetry of methyl methanesulfonate (MMS) in the germ cells of male mice has been investigated. The mice were injected i.p. with 100 mg/kg of [3H]MMS and methylations per sperm head, per deoxynucleotide, and per unit of protamine were then determined over a 3-week period. The methylations per sperm head paralleled the dominant lethal frequency curve for MMS, reaching a maximum of between 22 and 26 million methylations per vas sperm head 8-11 days after treatment. Methylation of sperm DNA was greatest at 4 h (the earliest time point studied) after treatment, with 16.6 methylations/10(5) deoxynucleotides. DNA methylation gradually decreased during the subsequent 3-week period. The methylation of germ-cell DNA did not increase in the stages most sensitive to MMS (late spermatids leads to early spermatozoa) and was not correlated with the dominant lethal frequency curve for MMS. However, methylation of protamine did increase in the germ-cell stages most sensitive to MMS, and showed an excellent correlation with the incidence of dominant lethals produced by MMS in the different germ-cell stages. The pattern of alkylation produced by MMS in the developing germ-cell stages of the mouse is similar to that found for EMS. However, for equimolar exposures, MMS alkylates the germ cells 5-7 times more than does EMS. Hydrolyzed samples of protamine from [3H]MMS-exposed animals were subjected to thin-layer chromatography and amino acid analysis. Both procedures showed that most of the labeled material recovered from the hydrolysates co-chromatographed with authentic standards of S-methyl-L-cysteine. The amino acid analyses showed an average of approximately 80% of the labeled material eluting with S-methyl-L-cysteine. The mechanism of action of both MMS and EMS on the developing germ cells appears to be similar. The occurrence of S-methyl-L-cysteine as the major reaction product in sperm protamine after MMS exposure supports our initial model of how dominant lethals are induced in mouse germ cells by these chemicals: Alkylation of cysteine sulfhydryl groups contained in mouse-sperm protamine blocks normal disulfide-bond formation, preventing proper chromatin condensation in the sperm nucleus. Subsequent stresses produced in the chromatin structure eventually lead to chromosome breakage, with resultant dominant lethality.

Role of 3-methyladenine-DNA glycosylase in host-cell reactivation of methylated T7 bacteriophage

Purified T7 phage, treated with methyl methanesulfonate, was assayed on four Escherichia coli K12 host cells: (1) AB1157, wild-type; (2) PK432-1, lacking 3-methyladenine-DNA glycosylase (tag); (3) NH5016, lacking apurinic endonuclease VI (xthA); (4) p3478, lacking DNA polymerase I (polA), the latter three strains being deficient in enzymes of the base excision repair pathway. For inactivation measured immediately after alkylation, phage survival was lowest on strains PK432-1 and p3478; for delayed inactivation, measured after partial depurination of alkylated phage, survival was much lower on strain p3478 than on PK432-1. These results demonstrate the important role played by 3-methyladenine-DNA glycosylase in the survival of methylated T7 phage. Quantitative analysis of the data, using the results of Verly et al. (Verly, W.G., Crine, P., Bannon, P. and Forget, A. (1974) Biochim. Biophys. Acta 349, 204-213) to correlate the dose with the number of methyl groups introduced into phage DNA, revealed that 5-10 3-methyladenine residues per T7 DNA constituted an inactivation hit for the tag mutant. Thus, 3-methyladenine may be as toxic a lesion as an apurinic site.

Effect of alkylation on the secondary structure of DNA

S1 nuclease hydrolysis and bezoylated naphthoylated DEAE-cellulose (BND-cellulose) chromatography have been used to demonstrate that alkylation of DNA by dimethyl sulfate at neutral pH leads to the production of partially denatured molecules under conditions where no significant depurination occurs. DNA was alkylated with increasing concentrations of the alkylating agent, and subjected to enzymatic degradation and binding to BND cellulose. An increasing degree of DNA hydrolysis and adherence to BND cellulose was seen. On hydroxyapatite chromatography the alkylated DNA still eluted at the position of double-stranded molecules suggesting the presence of partially denatured regions. The presence of salt had a preventive effect on such denaturation.

Depurination causes mutations in SOS-induced cells

Introduction of apurinic sites into phi X174 am3 DNA leads to loss of biological activity when measured in a transfection assay. For single-stranded DNA, approximately one apurinic site constitutes a lethal hit; for double-stranded (RFI) DNA, approximately 3.5 hits per strand are lethal. When the reversion frequency of am3 DNA is measured, no increase due to depurination is observed above the background level. However, a large increase in reversion frequency is observed when the same DNA is assayed by using spheroplasts derived from bacteria previously exposed to UV light. The results suggest that apurinic sites are impediments to a replicating DNA polymerase; however, nucleotides can be incorporated opposite these sites under SOS-induced conditions. We estimate the frequency of mutagenesis per apurinic site to be less than 1 in 1400 in normal spheroplasts and 1 in 100 in SOS-induced spheroplasts.

Mutagenicity, cytotoxicity and DNA crosslinking in V79 Chinese hamster cells treated with cis- and trans-Pt(II) diamminedichloride

The mutagenicity and cytotoxicity of cis- and trans-Pt(II) diamminedichloride (PDD) were examined in V79 Chinese hamster lung cells and compared with effects on DNA measured by alkaline elution. DNA--protein crosslinks and DNA interstrand crosslinks were detected following doses of cis-PDD which reduced cell survival 80--90% and which produced a mutant frequency of 3 X 10(-4) at the HGPRT locus. Equitoxic doses of trans-PDD were much less mutagenic than cis-PDD. At equitoxic doses, trans-PDD produced more DNA-protein crosslinking than did cis-PDD, but interstrand crosslinking for the two isomers was comparable. Hence, the interstrand crosslink could be the cytotoxic lesion produced by these Pt compounds whereas neither of these DNA lesions are necessarily mutagenic. The mutagenesis produced by cis-PDD could be due to crosslinks of a different type than those produced by trans-PDD or it may be due to monofunctional damage.

Participation of the intracellular enzymes in the control of mutational processes : Part 4: the role of UV-specific endonuclease and medium composition in the induction of genic mutations in Escherichia coli

The influence of UV-specific endonuclease and medium composition on the frequency and spectrum of genic mutations in Escherichia coli KI2 uvr (+) (with normal repair enzymes) and urv A6 (defective in UV-specific endonuclease) was studied. Mutations at the locus glu (gene controlling assimilation of glucose) were induced by ultra-violet irradiation and hydroxylamine treatment. To identify mutant colonies, triphenyl tetrazolium chloride (TTC) was added to the medium since it coloured the mutant colonies bright crimson and readily permitted distinction between pure mutant clones (complete mutations) and mixed clones (mosaic or sector mutations).A maximum mutation frequency after UV-irradiation was observed in E. coli uvr (+) cells but not in the E. coli uvr A6 strain. The curve of mutagenesis with a maximum was found in both studied strains after treatment by hydroxylamine which did not cause DNA damage recognized by UV-specific endonuclease.The highest frequency of mutations (at the point of maximum) in the series of experiments with enriched growth medium was almost 10 times higher than in the series of the experiments with poor medium.It was established that in bacteria with normal repair enzymes the frequency of complete mutations was higher than the frequency of mosaic mutations. It was also observed that the rate of UV-mutagenesis was higher in the case of E. coli uvr (+).The study of the distribution of mosaic mutant sectors in experiments with bacteria suspended in either a nutrient broth or a buffer during UV-irradiation revealed that the size of mutant sectors was rather variable and that the differences in the number of nucleoids per cell did not always determine the distribution of mutant sector sizes.

Recombinational repair of alkylation lesions in phage T4. I. N-methyl-N'-nitro-N-nitrosoguanidine

Treatment of phage T4-host adsorption complexes by MNNG increased recombination between two rII markers by about three-fold. Temperature sensitive mutants defective in genes 32, 46 and 47, which cause reductions in recombination at semirestrictive temperature, proved to be substantially more sensitive to MNNG at such temperatures than wild-type phage. In addition, the recombination defective mutants xm(uvsX) and y10(y) were sensitive to MNNG than wild-type, whereas mutants defective in genes 45 and denV, which are apparently not involved in recombination, were not MNNG sensitive. These findings suggest that a recombination pathway involving the products of genes 32, 46, 47, uvsX and y is employed in repairing MNNG-induced lethal lesions. This mechanism is effective in cells infected by single phage, implying post-replication recombinational repair between daughter chromosomes. MNNG-induced lesions are subjects to multiplicity reactivation, but mutants defective in genes 46 to 47 showed the same degree of multiplicity reactivation as wild-type phage. The gene 32 and gene 47 recombination defective mutants were tested for their effects of MNNG-induced reversion of an rII marker. No reduction in induced reversion was found. Thus, it appears that the postulated recombinational repair pathway employing the products of genes 32 and 47 does not contribute substanitally to induced mutagenesis.

Ethylation of DNA and protamine by ethyl methanesulfonate in the germ cells of male mice and the relevancy of these molecular targets to the induction of dominant lethals

The molecular dosimetry of ethyl methanesulfonate (EMS) in the germ cells of male mice has been investigated. The mice were injected i.p. with 200 mg/kg of [3H]EMS and the ethylations per sperm head, per deoxynucleotide, and per unit of protamine were then determined over a 2-week period. The ethylations per sperm head closely paralleled the dominant-lethal frequency curve for EMS, reaching a maximum of 5 to 6.5 million ethylations per vas sperm head at 8 to 10 days after treatment. Ethylation of sperm DNA was greatest at 4 h after treatment, with 5.7 ethylations/10(5) deoxynucleotides, and gradually decreased to 2.2 ethylations/10(5) deoxynucleotides at 15 days after treatment. The ethylation of sperm DNA did not increase in the germ-cell stages most sensitive to EMS, and was not correlated with the dominant-lethal frequency curve for EMS. However, ethylation of sperm protamine did increase in the germ-cell stages most sensitive to EMS, and showed an excellent correlation with the incidence of dominant lethals produced by EMS in the germ cells. A model is presented to explain, at a molecular level, how dominant lethals may be induced in mouse germ cells by EMS. Ethylation of cysteine sulfhydryl groups contained in mouse-sperm protamine could block normal disulfide-bond formation, preventing proper chromatin condensation in the sperm nucleus. Stresses in the chromatin structure could then eventually lead to chromosome breakage, with resultant dominant lethality.

Protein-associated DNA breaks in cells treated with adriamycin or ellipticine

The effect of intercalating agents on mammalian DNA in vivo was examined by the technique of alkaline elution. Adriamycin and ellipticine were found to produce large numbers of single-strand breaks. These breaks appeared to be intimately associated with protein to the extent that enzymatic deproteinization of the DNA was necessary to reveal the breaks. The frequency of breaks and cross-links increased with concentration and time of exposure to the drugs. These data suggest that DNA single-strand scission may be a feature common to intercalators. The association with a cellular protein is previously undescribed and suggests possible mechanisms for the strand scission.

Comparative mutagenicity of alkylsulfate and alkanesulfonate derivatives in Chinese hamster ovary cells

Mutation induction and cell killing produced by selected alkylsulfates and alkanesulfonates have been quantitated using the Chinese hamster ovary/hypoxanthine--guanine phosphoribosyl transferase (CHO/HGPRT) system. Dose--response relationships of cytotoxicity and mutagenicity are presented for two alkylsulfates [dimethylsulfate (DMS), diethylsulfate (DES)] and three alkyl alkanesulfonates [methyl methanesulfonate (MMS), ethyl methanesulfonate (EMS), and isopropyl methanesulfonate (iPMS)]. Under the experimental conditions employed, cytotoxicity decreased with the size of the alkyl group. DMS was more toxic than DES, and MMS was more toxic than EMS and iPMS. All agents produced linear dose--response of mutation induction: DMS was more mutagenic than DES, and MMS was more mutagenic than EMS and iPMS based on mutants induced per unit mutagen concentration. However, the following relative mutagenic potency was observed when comparisons were made at 10% survival: DES greater than DMS; EMS greater than MMS greater than iPMS.

Properties of the main endonuclease specific for apurinic sites of Escherichia coli (endonuclease VI). Mechanism of apurinic site excision from DNA

The main endonuclease for apurinic sites of Escherichia coli (endonuclease VI) has no action on normal strands, either in double-stranded or single-stranded DNA, or on alkylated sites. The enzyme has an optimum pH at 8.5, is inhibited by EDTA and needs Mg2+ for its activity; it has a half-life of 7 min at 40 degrees C. A purified preparation of endonuclease VI, free of endonuclease II activity, contained exonuclease III; the two activities (endonuclease VI and exonuclease III) copurified and were inactivated with the same half-lives at 40 degrees C. Endonuclease VI cuts the DNA strands on the 5' side of the apurinic sites giving a 3'-OH and a 5'-phosphate, and exonuclease III, working afterwards, leaves the apurinic site in the DNA molecule; this apurinic site can subsequently be removed by DNA polymerase I. The details of the excision of apurinic sites in vitro from DNA by endonuclease VI/exonuclease III, DNA polymerase I and ligase, are described; it is suggested that exonuclease III works as an antiligase to facilitate the DNA repair.

The binding of ethyl carbamate to DNA of mouse liver in vivo: the nature of the bound molecule and the site of binding

Ethyl carbamate, labelled at C1 with 14C, bound in vivo to liver DNA of intact and partially hepatectomised mice. Isotope (18O) enrichment was not detected in the oxygen of liver DNA of mice injected with [18O] ethyl carbamate, C2H5--18O--CO--NH2. This suggests that it was the ethyl group and not the ethoxy group which bound to DNA. Chromatographic analysis of acid hydrolysates of liver DNA from mice treated with [1-14C] ethyl carbamate provided no evidence of alkylation or other form of binding to purine or pyrimidine bases. On relatively mild acid hydrolysis the alkyl group remained bound to the "apurinic acid" fraction, while more vigorous hydrolysis lead progressively first to its separation as highly ionisable hydrophilic non-volatile compounds and then to its loss as a volatile compound. DNAase I followed by phosphodiesterase hydrolysis also split off the 14C-containing group as a volatile compound. The volatile compound was identified as ethanol. These results suggest that the alkyl group was bound as an ester to a phosphate group in the DNA chain. Results with DNA from partially hepatectomised mice did not differ from those with DNA from intact mice.

Role of apurinic sites in the resistance of methylated oligodeoxyribonucleotides to degradation by spleen exonuclease

The effect of introducing methyl groups into DNA substrates was studied by using the spleen exonuclease (EC 3.1.4.1), an enzyme which hydrolyses oligonucleotides in a sequential manner by splitting off 3'-phosphomononucleotides starting from the 5'-hydroxyl terminus. Analyses of oligodeoxyribonucleotide 3'-phosphate substrates after reaction in vitro with dimethyl sulphate demonstrated that the resultant methylation pattern differed from the previously found for native DNA, particularly with respect to the relative amounts of 1- and 3-methyladenine produced. Although after treatment with increasing amounts of dimethyl sulphate the substrate became progressively resistant to degradation by the exonuclease, the methylation products themselves were only partially responsible for the observed inhibition of enzyme activity. The incomplete degradation encountered was apparently due to the presence of apurinic sites, which arose as secondary lesions after the spontaneous release of the labile alkyl purines from the methylated substrate. Inhibition of enzyme activity appeared to be competitive, being characterized by constant values for apparent Vmax, and increased values for apparent Km. the interpretation of this, however, is complicated by the complex nature of the substrate, and these aspects are considered in some detail.

The nature of the alkylation lesion in mammalian cells

Methylating agents may produce as many as nine alkylated purine and pyrimidine adducts in DNA, as well as forming phosphotriesters and inducing apurinic sites and strand breaks. Although some of these products are formed in proportionately small amounts, there are sufficient sites affected in the DNA of a mammalian cell to make even the most minor product of potential biological significance. It is not possible to specify the exact reaction sites resulting in biological damage, but it is possible to quantitate the excisiion-repair of such damage both in the bulk of the DNA and at DNA growing points. Excision-repair can be measured in the bulk of the DNA by determining the specific activity of the NaCl eluate of a benzoylated naphthoylated DEAE-cellulose column of extracts of cells after treatment and incubation in the presence of hydroxyurea and labeled thymidine. The average number of nucleotides inserted per methyl methanesulfonate-induced methyl group is 0.1, per apurinic site is 9. Repair in growing-point regions after methyl methanesulfonate treatment occurs to approximately the same extent as in the bulk of the DNA.

Intermolecular linking and fragmentation of DNA by beta-propiolactone, a monoalkylating carcinogen

Brief exposure to beta-propiolactone (BPL) increases the sedimentation rate of purified Escherichia coli DNA in neutral and alkaline sucrose gradients. However, when electrophoresed in polyacrylamide-agarose gels, this BPL-treated DNA moves ahead of the control. Longer incubation with BPL gives rise to two new fractions, the first one sedimenting as a heterogeneous material of 6-8S, and the second one of very high sedimentation velocity. In acrylamide-agarose gels, the first fraction is again recovered in the 6-8S area, while the second fraction does not enter the gel at all. The DNA at this stage is hyperchromic in ultraviolet light suggesting that as much as 20% may be denaturated. Coliphage lambda DNA treated briefly with BPL and spread in a protein monolayer appears under the electron microscope as a rigid, extended molecule, up to 15% longer than the control DNA, and usually in compact, folded configurations suggesting intramolecular linking. After longer exposure, localized denaturation associated with single-strand breaks is observed. The single-stranded "whiskers" then interact with other DNA molecules, creating highly complex branched networks of single- and multi-stranded DNA. The possible relevance of these observations to the mechanisms involved in carcinogenesis and mutagenesis is considered.